Cryogenic coolers are used to cool devices, such as an infrared detector of a spacecraft, to cryogenic temperatures between about 40 degrees Kelvin to about 80 degrees Kelvin. For this purpose, a Stirling cycle expander is often used. Such a cryocooler can form part of a multi-stage cryocooler, also termed a two-stage expander, having a Stirling expander and a pulse tube expander. Examples of these systems are disclosed in U.S. Pat. Nos. 5,392,607, 5,412,951, 5,680,768, 6,167,707 and 6,330,800, the disclosures of which are incorporated herein by reference in their entireties.
Conventional cryocoolers include a motor used to drive a piston or displacer. Such motion can result in vibration of the cryocooler that can, in turn, disrupt operation of the cooled item. For example, when the cooled item is an optical detector system, such as a system that includes optics and/or a focal plane array, the performance degradation due to vibration attributable to the cryocooler module can reach unacceptable levels.
Attempts to limit the amount of vibration have included using dual opposed motors with an active vibration feedback and cancellation system. In this arrangement, the motors are placed in opposed orientations such that the moving mass driven by each motor is accelerated in opposite direction using, in an ideal implementation, identical forces. If the system is ideal, the net fore experienced by the cryocooler module would be zero.
Unfortunately, in practice, the dual opposing motor suffers from imperfections and/or inequalities in the motors, moving masses, suspension system stiffness and so forth. As a result, the cryocooler module experiences a non-zero total force and unacceptable levels of vibration can result. Therefore, the dual-opposed motor solution has been supplemented with an active feedback system. In that system, the net vibration output of the cryocooler module is sampled. For instance, load washers are placed in the load path between the cryocooler module and a mounting bracket and the load washers are used to detect the vibration of the cryocooler and provide a corresponding electrical signal. This signal is processed to produce a digital vibration-canceling waveform (or vibration trim signal) that is combined with (i.e., added to) a digital temperature control signal. The combined signal is amplified with a pulse width modulated (PWM) amplifier and the motor is driven in accordance with the amplified, combined signal.
The foregoing feedback control solution is limited by the minimum motor current that can be commanded as determined by the least significant bit available from the processor/servo loop. More particularly, the trim signal is much smaller in magnitude than the temperature control signal, which represents the motor's main drive parameter. Since the signal path leading to the main drive amplifier is of limited resolution, the relatively small trim signal is represented at best by a few of the combined signal's least significant bits. Therefore, the trim signal component of the signal delivered to the amplifier is very “rough” and an upper limit of its effectiveness to counter vibration is quickly encountered since the force required for vibration cancellation is smaller that what can be accurately represented in the signal path leading to the drive amplifier. For example, if the desired motor current is represented at the input to the amplifier as a twelve bit signal, the maximum possible number of discrete steps is 4096. If the maximum current for the example is ten amperes, then the current resolution that can be applied to the motor is 2.44 mA. If the typical force constant for the motor in the example is fourteen Newtons per Ampere (N/A), then the force resolution is about 34.16 milliNewtons (mN) (i.e., 2.44 mA times 14 N/A equals 0.03416 Newtons). As a result, vibration cannot be effectively controlled with any finer resolution than changing the force in steps of about 34 mN.
Vibration control using the foregoing solution is further hampered by other factors. For example, while PWM amplifiers are used for efficiency, they typically exhibit relatively high amounts of total harmonic distortion (THD) that can interfere with fine trim signals used for low-level vibration regulation. Also, if the number of time steps associated with each on/off period of the PWM amplifier is not sufficiently high enough, the resolution of the vibration cancellation can be degraded. Additionally, power MOSFETs used in an output section of the PWM amplifier have a fairly long turn-on time with respect to the duty cycle time step size, which can lead to non-negligible crossover distortion.
Accordingly, there is a need in the art for a cryocooler with improved vibration characteristics.